1. Field of the Invention
This invention relates generally to secure communication systems, and more particularly, to a high frequency covert communication IFF system which can receive low power narrow bandwidth signals.
2. Description of the Related Art
There has been a long-standing concern with the security of communication systems employing electromagnetic radiation. This concern has arisen due to the relative ease with which most transmitted communications signals can be intercepted by unauthorized persons. While undesirable in any situation, this is most critical, and life threatening, in modern warfare. While many countermeasures such as cryptography have been developed to alleviate these problems, even where the transmitted message cannot be deciphered by an enemy, the fact that the enemy can receive a signal originating from a particular point creates problems. This is because the signal may give sufficient information about the location and velocity of the transmitter to permit the enemy to destroy the detected transmitting source, whether it be stationary or on a vehicle based on land, sea or air.
These problems are particularly acute in Identification-Friend-or-Foe (IFF) systems. IFF and similar transponder systems are traditionally used to allow a transponder in an airplane or other vehicle to respond in a particular way to a special signal from an interrogating ground station or aircraft. Information of this nature is vitally important to a military aircraft-intercept station desirous of knowing whether an approaching aircraft is friend or foe. Similar systems are also found in air traffic control interrogation systems.
In the typical interrogation transmitter system, an interrogation request is first transmitted. This request comprises a coded signal or message consisting of a train of pulses spaced along in time. This coded signal is received by the IFF transponder and analyzed. If the pulses are of the correct predetermined frequency, amplitude and spacing, a predetermined signal would be produced by the transponder. The expected response will then notify the interrogator that the recipient was a friend and not a foe. On the other hand, should no response or an incorrect response be sent, the interrogating system may take that to mean that target is a foe and a decision can then be made as to whether to initiate an attack upon it.
It is important that communications in systems such as IFF's be covert and secure for many reasons. For example, one problem with IFF systems is that since the system radiates a signal on demand (or, in some systems, continuously), if this radiated signal is detected by an enemy it can be used to locate and destroy its source. The mere fact that each side of the IFF system is transmitting information could allow an enemy to track, locate and destroy the sources of the signals. In some cases where a system responds on demand to an interrogation, an enemy could transmit a signal mimicking the interrogation signal to the receiver and use the response as a way to locate it. Part of the problem with IFF systems is that a relatively strong signal must be transmitted by both sides to enable it to be detected at relatively great distances. Yet, lowering of power of the signals would also require a corresponding increase in sensitivity of receivers, which also would significantly reduce the signal-to-noise ratio to the point to where the signal could not be detected.
Another problem with a lack of covert communication in IFF systems may result where an enemy is able to listen to both the interrogation and the responding signal and then configure its own aircraft to respond similarly. In this way, even enemy targets would appear as friend when interrogated and would likely not be attacked.
Outside of the IFF environment, there are many situations, both military and non-military, where it is desired to permit covert communications between two parties wherein the existence of the communication cannot be detected by outsiders beyond a limited physical distance. For example, there is a need for two aircraft in the same flight to be able to engage in communications between each other while maintaining electronic communication silence beyond a limited range.
In addition, while IFF systems are designed to prevent fratricide, equipment and/or human errors do arise which can allow such events to occur. This can happen due to a variety of causes. These causes may include: improper visual target identification prior to a missile launch; or non-operational or improperly set IFF equipment for beyond visual range engagement. In the case of homing missiles, fratricide can occur due to improper missile lock-on or in-flight reacquisition due to a confused tactical situation such as could exist during a close "furball" engagement. False target identification can also result when both friend and foe utilize identical-type aircraft due to changing political situations.
Thus, it would be desirable to provide an improved method of communication through electromagnetic radiation which is secure, covert and undetectable beyond a very limited distance. It would also be desirable to provide an improved or supplementary IFF system which does not rely on communications that can be readily detected by an enemy. It would further be desirable to improve IFF systems so that they are foolproof and more covert by providing more secure and covert communications. It would also be desirable to provide a communications system which can operate at extremely low power while achieving a satisfactory signal/noise ratio.
Furthermore, it would be desirable to provide an improved IFF system which overcomes the above-discussed situations which can result, and have in the past resulted, in fratricide. Thus, it would be desirable to provide a back-up IFF system which provides a fail-safe mechanism in the event that a conventional IFF system fails.